Air foil bearings that use air as a lubricant for a spinning shaft have become popular in small high speed turbomachinery, such as air cycle machines, small turbo-compressors/blowers, and micro gas turbines. Such bearings typically comprise one or multiple smooth top foils that are supported by an elastic support structure, such as a corrugated bump foil, that provides stiffness and damping.
The load capacity of foil bearings, which is measured at their full design speed, may be large enough for the foil bearings to be considered for applications beyond their current size limits. However, this load capacity is achieved because of the hydrodynamic action at full speed, which diminishes as the shaft slows down. The load capacity during start up and shut down relies only on the wear resistance of the materials of the shaft and top foil, or coatings on these components. Therefore, the average static bearing pressure (static load divided by diameter times length) of the foil bearings adopted in most commercial applications are much lower than the load capacity of the bearing measured at the design speed to enable many start/stop cycles without failure of the bearing or shaft, or their coatings.
Although surface coatings help to reduce the wear and friction to some extent, dry rubbing is a thermodynamically irreversible process that inevitably causes wear. Hybrid foil-magnetic bearings are one option to avoid the dry rubbing of the large foil bearings. However, the foil bearing is not the main bearing in such applications and acts only as a mechanical catcher bearing during electric system failure. Thus, the foil bearings may not survive many emergency stops. Alternative hybrid foil bearings that add hydrostatic injection to conventional foil bearings may be a more viable solution. Hybrid foil bearings combine the inherent hydrodynamic pressure of conventional foil bearings with the hydrostatic lift generated by pressurized gas (e.g., air) injected through orifices attached to the top foil to minimize or even eliminate dry rubbing during starts and stops. The hydrostatic injection can be turned off once the rotor reaches lift-off speed or it can be maintained as the hydrostatic injection is an excellent cooling mechanism. Needed, however, are hybrid foil bearing designs that are suitable for commercial applications.